As is generally known, a copying machine of this type records an image read out of a manuscript onto a recording paper by executing a series of processes such as:
(1) Photosensitivity is provided by charging a photosensitive substance. PA1 (2) An electrostatic latent image is produced by exposing the electrostatic substance to an optical image. PA1 3) The electrostatic latent image is developed with a toner. PA1 (4) The developed image is transferred to a recording paper. PA1 (5) The photosensitive substance is cleaned.
Furthermore, in a polychromatic copying machine, a polychromatic print that is the image of the manuscript is obtainable by means of color separation of the image of the manuscript, by repeatedly performing a series of processes of charging, exposure, development, transfer and cleaning as described above for every color separated image, and by forming by superposition the images in respective separated colors on the same recording paper.
In such a monochromatic or polychromatic copying machine, in order to have both the positional relationship between the manuscript picture image and the copied picture image, the positional relationship between respective colors coincide with each other, it is required to have the picture image scanning initiation timing of the optical scanning mechanism, which moves along the manuscript picture image surface. In other words the position where forming of the electrostatic latent image on the photosensitive substance is initiated and the position where transfer is initiated on the recording paper coincide exactly.
Therefore, in the copying machine of this type, it is required for a light source, a movable mirror, a photosensitive drum and a transfer drum, etc. to be driven exactly according to a predetermined timing so as to form the picture image. Accordingly, a control unit for controlling these positional relationship under the driving state is provided.
FIG. 24 is a schematic block diagram showing the structure of a conventional polychromatic copying machine. In the Figure, a manuscript table 102 is mounted on the upper surface of a main body 101, and a scan unit 103 is provided below this manuscript table 102. The scan unit 103 consists of a lamp 104, first and second mirrors 105 and 106, a filter lens unit 107, third and fourth mirrors 108 and 109, and so forth. The lamp 104 and the first mirror 105 are integrated :n a body so as to be movable in the directions A and B shown in the drawing. Furthermore, the second mirror 106 is constructed so as to move, according to the movement of the lamp 104 and the first mirror 105, at 1/2 of the speed of the movement.
In the copying operation, when the lamp 104 and the first mirror 105 are moved first in the direction shown with an arrow mark A, an optical image is irradiated onto the surface of a photosensitive drum 111, which is rotated clockwise. In this case, the filter lens unit 107 has been changed over so as to transmit the light having a color other than yellow color, and further, the photosensitive drum 111 has been charged by a charger 112. Therefore, said optical image becomes an electrostatic latent image corresponding to yellow color in the manuscript on the surface of the photosensitive drum 111. Then, yellow toner is deposited on this electrostatic latent image by means of a developing part 113. As a result, a toner image in yellow color is formed on the photosensitive drum 111.
On the other hand, the blank form fed from a blank form cassette 114 is wound a round a transfer drum 115, which rotates counterclockwise, and conveyed between the photosensitive drum 111 and a transfer drum 115. As a result, the abovementioned yellow toner image is transferred onto the blank form on the transfer drum 115. Then, the surface of the photosensitive drum 111 is cleaned in consecutive order by means of a cleaning unit 116 from the portion where transfer has been completed.
After the transfer of the yellow toner image is completed as described above, the filter lens unit 107 is changed over in the next place so as to transmit any color other than magenta color, and a developing part 117 for magenta color is selected at the same time, followed by the similar transfer operation as described above. Thereafter, the filter lens unit 107 is changed over so as to transmit any color other than cyanogen color, and a developing part 118 for cyanogen color is selected at the same time, thus performing similar transfer operation as described above. Then, when the transfer of three primary colors is completed, a composite image in yellow, magenta, and cyanogen colors is formed on the surface of the blank form on the transfer drum 115. Next the blank form on the transfer drum 115 is conveyed to a fixing unit 122 with a belt 121, and the color image formed on the blank form surface by means of this fixing unit 122 is securely fixed onto the blank form. Then, the blank form completed with fixing is ejected to a tray 123, thus completing a series of color copying operation.
FIG. 25 is a perspective view showing the outline of a position control mechanism of each movable part in the copying machine described above. The reference numeral 131 shown in the drawing is a chain with which the driving force of a motor (not shown) transmitted. Chain 131 is engaged with a sprocket 133. Reference numeral 132 denotes a shaft on which the sprocket 133 and a gear 134 are mounted with a common shaft center, and 135 reference numeral denotes a shaft on which the transfer drum 115 and a gear 136 are mounted. In abovementioned structure, when the sprocket 133 is rotated, the gear 134 and the photosensitive drum 111 are also rotated, and the gear 136 engaged with the gear 134 is rotated at the same time, which causes the shaft 135 to be rotated. With this, the transfer drum 115 is rotated. In this case, the pitch diameters of gears 134 and 136 are made to be the same. As the result, the photosensitive drum 111 and the transfer drum 115 rotate in reverse directions, at the same speed and synchronously with each other. Furthermore, on the transfer drum 115, the position of winding round the blank form is always controlled fixed by means of pawls 137 for controlling the position to wind round the blank form.
On the other hand, a pulley 142 is supported by the shaft through a bearing 141, and a movable pawl (not shown), which is driven by a solenoid, etc, is provided on the side of the pulley 142. When this pawl is driven and engaged with a pin 143 provided on the sprocket 133, the rotation of the shaft 132 is conveyed to the pulley 142, thereby to rotate the pulley 142 synchronously with the photosensitive drum 111 keeping a predetermined relationship with same. Then, the rotation of the pulley 142 is conveyed to a pulley 148 through a wire 144, and the rotation of this pulley 148 is conveyed to the scan unit 103 through shaft, pulley and wire, etc. As the result, when the pulley 142 is rotated, the lamp 104, etc. are moved in the direction shown by the arrow mark A corresponding to the rotation of the photosensitive drum 111. If the driving pawl slips off the pin 143, the 104, etc. are returned in the direction shown by the arrow mark B by means of the energizing force of a spring not shown.
According to abovementioned structure, since the scan unit 103 and the photosensitive drum 111 are mechanically interlocked with each other, the position of the electrostatic latent image formed on the photosensitive drum 111 becomes fixed. Moreover, since the photosensitive drum 111 and the transfer drum 115 rotate synchronously and in reverse directions with each other, and the position of winding the blank form a round the transfer drum 115 is fixed, positions of images in each color transferred on the blank form coincide with one another. As the result, color copying by process color printing is performed without causing color shear.
However, once a shear of positions of images in each color transferred onto the blank form occurs, color shear happens, resulting in an imperfect finished result, Accordingly, it is necessary to control very exactly the driving position relationship among the scan unit 103, the photosensitive drum 111, and the transfer drum 115.
In the abovesaid control unit, however, the whole interlocking of movable parts is performed mechanically. Therefore, it may happen sometimes that initial positions of each part of movable parts are varied by secular change, etc. As the result, there has been such a problem that the position of forming the electrostatic latent image is shifted, causing color shear to happen.
In order to prevent such color shear, etc., a unit has been proposed, wherein driving motors are provided for the scan part provided movably on a predetermined straight line track, wherein the rotating photosensitive drum keeps a predetermined relationship with the movement of this scan part, and wherein the rotating transfer drum keeps a predetermined relationship with this photosensitive drum, respectively, and wherein the structure is constituted in such a way that said photosensitive drum and said transfer drum are driven individually by means of abovementioned driving motors. Pulse encoders are provided for detecting rotational quantity of each of the abovementioned driving motors to control each of said driving motors individually based on the output of this pulse encoder.
According to such a unit, since color shear can be securely prevented from occurring and since the scan part, the photosensitive drum, and the transfer drum are interlocked with an electrical timing, such a unit has the following advantages: (1) no secular change occurs in point of the positional relationship, (2) reduced/enlarged copies of manuscripts are easily made available without requiring complicated mechanical mechanism, and (3) improvement of the copying efficiency may be aimed at by adopting a short scan, etc.
In abovementioned structure, the optical scanning mechanism is returned to the stop position by means of the energizing force of a spring, but some units are constructed in such a way that a counter that outputs the present position signal of the optical scanning mechanism by means of up-count and down-count of a rotation pulse synchronizing with the moving speed of the optical scanning mechanism is provided, and the optical scanning mechanism is made to move to the operation terminating position by the present position signal shown with the output of said counter, and is returned to the stop position thereof thereafter.
However, problems have occurred when the counter output is smaller than when said counter is in suspension at a specified stop position due to noise, etc. when the optical scanning mechanism is returned to the stop position thereof, such a state is produced that a motor as the motive power source is still controlled under accelerated condition even after the optical scanning mechanism passes the specified stop position and has reached the position of the stopper, and troubles such as burning of motor windings and driving circuits thereof are induced, thereby making the maintenance operation thereof very difficult.
Further, in a conventional structure as described above, acceleration/deceleration control of the rotation of the transfer drum is performed so that the point of the transfer paper and the electrostatic latent image forming initiation point can be made to coincide with each other by performing acceleration/deceleration control of the transfer drum. Therefore, if an abnormal matter occurs in a rotary encoder employed for the purpose of controlling the grip timing of the transfer paper, the transfer initiating point and the latent image forming initiation point slip off and the positional relationship with the manuscript picture image is dislocated. In particular, since the electrostatic latent images are formed three times in total in a polychromatic copying machine, a copied picture image faithful to the manuscript picture image is not available because of color shear. Moreover, the motor, which is the power source for the photosensitive subject, is caused to accelerate condition even when the end timing of the transfer cycle is reached. Therefore, troubles such as burning of motor windings or driving circuits thereof are generated, making the maintenance operation very difficult thereafter
Still further, there is such a problem that, when it is arranged that the movable optical system, the photosensitive substance, and so forth, are controlled by independent servo loops, respectively, if an abnormal matter occurs in any of those servo loops, diagnosis becomes difficult because each of servo loops is not connected in a mechanical relationship.
Still further, in the structure described above, the movable optical system, which scans the manuscript picture image, the photosensitive substance, and the transfer drum are driven independently by means of individual servo loops. For example, for the transfer drum, there are provided a rotary encoder that generates a pulse signal synchronizing with the rotation of the transfer drum, and a preset counter that rotates the transfer drum in accordance with the difference between a pulse train corresponding to the target value cf the rotation quantity of the transfer drum. The pulse signal are provided in the servo loop, thereby to rotate the transfer drum until the count value of the preset counter becomes zero.
However, a gripper for gripping the transfer paper is mounted on the circumferential surface of the transfer drum. Additionally, a release cam, which peels off the transfer paper that is completed with transfer, is arranged as it were seeing the circumferential surface. Therefore, if the gripper and the release cam engage each another, or the gripper engages with other protruded part of the frame because of some causes, the rotation of the transfer drum presents locked condition. Then, since the pulse signal, which is synchronous with the rotation, will no longer be output, causing problems such as the counter value of the present counter is not reduced and, the applied voltage of the motor, which is the motive power source of the transfer drum, continues to be under accelerated condition. Furthermore, troubles such as burning of windings and driving circuits thereof are caused.
Still further, the abovementioned configuration, it has been arranged in such a way that the positional error of the gripper is detected at the scan initiation timing of the picture image, and acceleration/deceleration control of the transfer drum is executed immediately based on said positional error. As the result, misgripping occurs when the rotation speed of the transfer drum is varied immediately before the gripping operation of the transfer paper.
Still further, in the abovementioned configuration, there have been such problems that, when an abnormal matter has occurred in the signal path of the pulse encoder or a noise is mixed in, the interlocking relationship between the photosensitive drum and the scan unit or the interlocking relationship between the photosensitive drum and the transfer drum collapses, the forming initiation position of the electrostatic latent image becomes unstable, the positional relationship with the manuscript picture image is shifted, a specific color is missing and a copied picture image faithful to the manuscript picture image becomes unobtainable particularly in a polychromatic copying machine wherein, electrostatic latent images are formed three time. Moreover, the motor, which is the motive power source of the photosensitive substance, continues to be accelerated even at the termination timing of the transfer cycle, thus causing troubles such as burning of motor windings and driving circuits thereof and making the maintenance operation very difficult thereafter.
Still further, in the abovementioned configuration, the photosensitive substance is started in such a way that the time is measured with the start initiation timing of the optical scanning mechanism as the initiation point, and the electrostatic latent image in the next color is formed by starting the optical scanning mechanism again when the measured time reaches the copy initiation time for that next color.
In this case, however, the synchronous relationship between the optical scanning mechanism and the photosensitive substance is dislocated in every copy cycle for respective colors by means of nonuniformity of the rotation period of the photosensitive substance. Such dislocation is accumulated and causes even bigger nonuniformity in shade for each color.
Still further, in the configuration described above, acceleration/deceleration control of the transfer drum is performed so that the transfer initiation point and the latent image forming initiation point may coincide with each other with the grip timing signal of the transfer paper which is output synchronously with the rotation of the drum as the reference.
In fact, the accuracy of a sensor which generates said timing signal is low, and further, usually a l:m gear intervenes between the motor as the motive power source and the transfer drum. As a result, the transfer initiation point and the latent image forming initiation point slip off from each other.
On the circumferential surface of the transfer drum, a plastic net is formed in the length corresponding to the maximum length of the transfer paper so as to attract the transfer paper with static electricity. If the picture image forming area of the photosensitive drum stops at the portion of this plastic net, abnormal transfer, viz., so-called deletion is generated at the time of transfer. Therefore, it is required to perform control to stop the photosensitive drum and the transfer drum so that the electrostatic latent image forming area of the photosensitive drum and the plastic net do not accord with each other. Besides, such a relationship must also be returned to the normal positional relationship after the relationship between both is shifted due to paper jam.
However, the starting positional relationship between the photosensitive substance and the transfer drum have been heretofore adjusted by a Control Enable signal only when an abnormal matter such as a paper jam occurred. Accordingly, that the positional relationship between the photosensitive substance and the transfer drum is left as is, even if said relationship is shifted for some reason until an abnormal matter occurs, thus lowering the picture quality.
Still further, the control for returning the optical scanning mechanism to the stop position thereof has heretofore depended on the energizing force of a spring only. Therefore, the stop position of the optical scanning mechanism is shifted in every copy cycle due to the state variation of a motive power conveying mechanism, etc., which makes the running time of the optical scanning mechanism different when copying is initiated again. Thus, the positional relationship between the manuscript picture image and the copied picture image or the positional relationship between respective colors no longer coincides, causing such problems as color shear in a polychromatic copying machine and a deterioration of picture of quality.
Still further, in the abovementioned configuration, the rotation of the photosensitive substance, the transfer drum, and so forth is controlled based on the pulse signals from a pulse generator (a rotary encoder) mounted on the rotation shaft of each rotating body.
However, there have been problems that, when some abnormal matters occur in the pulse generator or the signal path thereof, the motor, which is the motive power source of the photosensitive substance, is accelerated even after the termination timing of the copying cycle is reached, causing troubles such as burning of motor windings and driving circuits thereof and making the maintenance operation very difficult thereafter.